1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrographic copying machine, a facsimile machine and a printer, and more particularly, to an improvement of a conveying guide for conveying a sheet onto which an image has been transferred.
2. Description of the Prior Art
In an image forming apparatus in which a toner image formed on the surface of a photoreceptor is transferred onto a transfer sheet, generally, a charging section, an exposure section, a development section and a transfer section are arranged in this order in a direction of rotation of the photoreceptor drum which rotates in one direction. After charged at the charging section, the drum surface is exposed at the exposure section to thereby form an electrostatic latent image in accordance with image information. Then, at the development section, charged toner is attached to the electrostatic latent image to develop the latent image into a toner image, and at the transfer section, the toner image formed on the drum surface is transferred onto the transfer sheet conveyed to the transfer section in synchronism with the rotation of the drum.
Referring to FIG. 1, there is shown the structure of a conventional transfer section. In the transfer section, a transfer roller 52 serving as a charge supplying means is provided below a photoreceptor drum 51 so as to face to the photoreceptor drum 51. Between the drum 51 and the transfer roller 52, a sheet path R' is provided so that a transfer sheet P is conveyed in a direction corresponding to the direction of rotation of the drum 51 shown by an arrow r'.
The sheet P conveyed along the path R' enters the gap between the drum 51 and the transfer roller 52, and then, electrostatically adheres to a transfer area on the drum surface. To the transfer roller 52, a voltage is applied of a polarity reverse to that of the toner image on the drum surface. The charged toner forming the toner image is attracted by the voltage, so that the toner image is transferred onto the surface of the sheet P. After the transfer, the sheet P is separated from the drum surface, for example, by curvature separation, and conveyed to a non-illustrated fixing section along a conveying guide 53 provided in the downstream side of the gap between the drum 51 and the transfer roller 52.
In this arrangement, normally, the conveying guide 53 supports the sheet P at its reverse surface, and an upper surface 53a along which the sheet P is conveyed is made of a highly insulating synthetic resin material. The upstream side half of the upper surface 53a gently declines in the sheet conveying direction. The downstream side half thereof is substantially level. On the upper surface 53a of the conveying guide 53, a plurality of guide ribs 54 are formed along the path R' to be parallel to one another. The contact friction is reduced since the reverse surface of the sheet P is partly supported by the guide ribs 54. As a result, the sheet P is smoothly conveyed toward the downstream side.
Conventionally, the height of the upstream side end of the upper surface 53a of the conveying guide 53 which is closest to the drum 51 and to the transfer roller 52 is substantially the same as the height of the end of the sheet which has just been separated from the drum surface on the path R', so that the end of the sheet separated from the drum surface is smoothly moved onto the upper surface 53a of the conveying guide 53.
However, in the conventional arrangement where the height of the upstream side end of the upper surface 53a of the conveying guide 53 is substantially the same as the height of the path R', the sheet P is pressed and rubbed against the guide ribs 54 formed on the conveying guide 53 just after the toner image is transferred thereonto, so that the rubbed portion is highly charged due to friction.
At this time, since the sheet P has just been charged at the time of transfer, the charge thereon is substantially maximum. Under such a condition, the toner forming an un-fixed image on the sheet P is strongly attracted by the charged areas of the sheet P rubbed against the guide ribs 54. As a result, traces of the ribs 54 are formed on the image. When the electric resistance of the resin material of which the conveying guide 53 is made is lower than the resistance of the sheet P, rib trances are formed more conspicuously since the charge is accumulated more easily in the conveying guide 53.
In addition, the portion of the sheet P adhering to the drum surface always receives a reaction force from the preceding part of the sheet separated from the drum surface since the preceding part is strongly rubbed against the conveying guide 53. When the image transfer onto the rear end of the sheet P is performed, the area of the sheet P adhering to the drum surface is small, so that the rear end of the sheet P is slightly shifted by the force received from the preceding part. As a result, the image is transferred onto an incorrect position in the rear end of the sheet P.